System and method for performing a fast handover in a broadband wireless access communication system

ABSTRACT

A broadband wireless access (BWA) communication system includes a mobile subscriber station (MSS), a serving base station (BS) with which the MSS performs communication, and a plurality of neighbor BSs. The serving BS transmits, to the MSS, a handover request message including information on at least one recommended BS to which the MSS can perform a handover, among the neighbor BSs, and indication information indicating a fast handover, and transmits, to each of the recommended BSs, a handover confirm message indicating that the MSS will perform the fast handover.

PRIORITY

This application is a Continuation of U.S. application Ser. No.11/125,028, filed on May 9, 2005 and claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Officeon May 7, 2004 and assigned Serial No. 2004-32394, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a broadband wireless accesscommunication system, and in particular, to a system and method forperforming a fast handover of a mobile subscriber station, initiated bya handover request of a serving base station.

2. Description of the Related Art

Research into a 4^(th) generation (4G) communication system, which isthe next generation communication system, is currently being conductedto provide users with services having various Qualities-of-Service(QoSs) at a transfer rate of about 100 Mbps. More particularly, activeresearch into the 4 G communication system is being carried out tosupport high-speed services for guaranteeing mobility and QoS in abroadband wireless access (BWA) communication system, such as a wirelessLocal Area Network (LAN) system and a wireless Metropolitan Area Network(MAN) system. Conventional communication systems include an Institute ofElectrical and Electronics Engineers (IEEE) 802.16a communication systemand an IEEE 802.16e communication system.

The IEEE 802.16a and IEEE 802.16e communication systems use anOrthogonal Frequency Division Multiplexing (OFDM) scheme and/or anOrthogonal Frequency Division Multiple Access (OFDMA) scheme in order tosupport a broadband transmission network for a physical channel of thewireless MAN system. The IEEE 802.16a communication system considersonly a state in which a subscriber station (SS) is located in a fixedposition, i.e., mobility of an SS is never taken into consideration, anda unicell structure. However, the IEEE 802.16e communication systemconsiders mobility of an SS in the IEEE 802.16a communication system,and accordingly, in the IEEE 802.16e communication system, the SS iscalled a mobile subscriber station (MSS).

FIG. 1 is a diagram schematically illustrating a conventional IEEE802.16e communication system. Referring to FIG. 1, the IEEE 802.16ecommunication system has a multicell structure, i.e., a cell 100 and acell 150, and includes a base station (BS) 110 managing the cell 100, aBS 140 managing the cell 150, and a plurality of MSSs 111, 113, 130,151, and 153. Signal exchange between the base stations 110 and 140 andthe MSSs 111, 113, 130, 151, and 153 is achieved using the OFDM/OFDMAscheme.

The MSS 130 is located in a boundary region of the cell 100 and the cell150, i.e., a handover region. If the MSS 130, while exchanging signalswith the BS 110, moves in the direction of the cell 150 managed by theBS 140, its serving BS changes from the BS 110 to the BS 140.

FIG. 2 is a signaling diagram illustrating a handover process initiatedat the request of an MSS in a conventional IEEE 802.16e communicationsystem. Referring to FIG. 2, a serving BS 210 transmits a MobileNeighbor Advertisement (MOB_NBR_ADV) message to an MSS 200 in Step 211.A format of the MOB_NBR_ADV message is shown in Table 1.

TABLE 1 Syntax Size Notes MOB_NBR_ADV_Message_Format( ) { ManagementMessage Type = 49 8 bits Operator ID 24 bits  Unique ID assigned to theoperator N_NEIGHBORS 8 bits For (j=0; j<N_NEIGHBORS; j++) { NeighborBS-ID 48 bits  Physical Frequency 32 bits  Configuration Change Count 8bits Incremented each time the information for the associated neighborBS has changed. Hysteresis threshold 8 bits MAHO report period 8 bitsTLV Encoded Neighbor information Variable TLV specific } }

As shown in Table 1, the MOB_NBR_ADV message includes a plurality ofinformation elements (IEs), i.e., a Management Message Type indicating atype of a transmission message, an Operator ID indicating a networkidentifier (ID), an N_NEIGHBORS indicating the number of neighbor BSs, aNeighbor BS-ID indicating IDs of the neighbor BSs, a Physical Frequencyindicating a physical channel frequency of the neighbor BS, aConfiguration Change Count indicating the number of changes inconfiguration, a Hysteresis threshold indicating hysteresis information,a MAHO (Mobile Assisted HandOver) report period indicating a period forwhich an average carrier-to-interference and noise ratio (CINR) value ofa neighbor BS is reported, and a TLV (Type/Length/Value) EncodedNeighbor Information indicating other information related to theneighbor BS.

The MSS 200 can acquire information on neighbor BSs by receiving theMOB_NBR_ADV message. If the MSS 200 desires to scan CINRs of pilotchannel signals transmitted from neighbor BSs and the serving BS 210, ittransmits a Mobile Scanning Interval Allocation Request (MOB_SCN_REQ)message to the serving BS 210 in Step 213. A format of the MOB_SCN_REQmessage is shown in Table 2.

TABLE 2 Syntax Size Notes MOB_SCN_REQ_Message_Format( ) { ManagementMessage Type = 50 8 bits Scan Duration 12 bits  Units are framesreserved 4 bits }

As shown in Table 2, the MOB_SCN_REQ message includes a plurality ofIEs, i.e., a Management Message Type indicating a type of a transmissionmessage, and a Scan Duration indicating a scanning duration for whichthe MSS 200 desires to scan CINRs of pilot channel signals received fromthe neighbor BSs. Because a time at which the MSS 200 makes a scanrequest is not directly related to a CINR scanning operation for thepilot channel signals, a detailed description thereof will not be givenherein.

Upon receiving the MOB_SCN_REQ message, the serving BS 210 includesinformation based on which the MSS 200 will perform scanning in a MobileScanning Interval Allocation Response (MOB_SCN_RSP) message with ScanDurations≠0, and transmits the MOB_SCN_RSP message to the MSS 200 inStep 215. A format of the MOB_SCN_RSP message is shown in Table 3.

TABLE 3 Syntax Size Notes MOB_SCN_RSP_Message_Format( ) {  ManagementMessage Type = 51  8 bits  CID 16 bits basic CID of the MSS  Duration 12bits in frames  Start Frame  4 bits }

As shown in Table 3, the MOB_SCN_RSP message includes a plurality oflEs, i.e., a Management Message Type indicating a type of a transmissionmessage, a Connection ID (CID) indicating a CID of the MSS 200 thattransmitted the MOB_SCN_REQ message, a Scan Duration, and a Start Frameindicating a time at which a scanning operation starts. The ScanDuration indicates a scanning duration for which the MSS 200 performsthe pilot CINR scanning, and if the Scan Duration is set to ‘0’ (ScanDuration=0), it indicates that the scan request of the MSS 200 isrejected by an SS.

Upon receiving the MOB_SCN_RSP message including the scanninginformation, the MSS 200 performs CINR scanning on the pilot channelsignals received from the serving BS 210 and neighbor BSs acquiredthrough reception of the MOB_NBR_ADV message according to parameters,i.e., Scan Duration, included in the MOB_SCN_RSP message in Step 217.

After completing CINR scanning on the pilot channel signals receivedfrom the neighbor BSs and the serving BS 210, the MSS 200 determines ifit should change its current serving BS to a new serving BS, which isdifferent from the serving BS 210, in Step 219.

When the MSS 200 determines to changes its current serving BS, ittransmits a Mobile Subscriber Station Handover Request (MOB_MSSHO_REQ)message to the serving BS 210 in Step 221. Herein, a new BS other thanthe serving BS to which the MSS 200 currently belongs, i.e., a possiblenew serving BS to which the MSS 200 will be handed over, will bereferred to as a “target BS.”

A format of the MOB_MSSHO_REQ message is shown in Table 4.

TABLE 4 Syntax Size Notes MOB_MSSHO_REQ_Message_Format( ) {   ManagementMessage Type = 53 8 bits   For (j=0; j<N_Recommended; j++) { N_Recom-mended can be derived from the known length of the message     NeighborBS-ID 48 bits      BS CINR mean 8 bits     Service level prediction 8bits   }  Estimated HO start 8 bits The estimated HO time shall be thetime for the recommended target BS. }

As shown in Table 4, the MOB_MSSHO_REQ message includes a plurality ofIEs, i.e., a Management Message Type indicating a type of a transmissionmessage and the scanning results acquired by the MSS 200. In Table 4, anN_Recommended indicates the number of neighbor BSs that transmittedpilot channel signals of which CINRs are higher than or equal to apredetermined CINR, as a result of CINR scanning on the pilot channelsignals from the neighbor BSs by the MSS 200. That is, the N_Recommendedindicates the number of recommended neighbor BSs to which the MSS 200can be handed over.

The MOB_MSSHO_REQ message also includes a Neighbor BS-ID indicating IDsof neighbor BSs indicated by the N_Recommended, a BS CINR mean, whichindicates an average CINR for pilot channel signals from the neighborBSs, a Service level prediction indicating a service level predicted tobe provided to the MSS 200 by the neighbor BSs, and an Estimated HOstart indicating a time at which the MSS 200 will start handover.

Upon receiving the MOB_MSSHO_REQ message transmitted by the MSS 200, theserving BS 210 detects a list of candidate target BSs to which the MSS200 can be handed over, from N_Recommended information in the receivedMOB_MSSHO_REQ message in Step 223. Herein, the list of candidate targetBSs to which the MSS 200 can be handed over will be referred to as a“candidate target BS list,” and it will be assumed in FIG. 2 that thecandidate target BS list has a first target BS 220 and a second targetBS 230. The candidate target BS list can also include a plurality oftarget BSs, in addition to the two target BSs.

The serving BS 210 transmits HO_PRE_NOTIFICATION messages to the targetBSs included in the candidate target BS list, i.e., the first target BS220 and the second target BS 230 in Steps 225 and 227. A format of theHO_PRE_NOTIFICATION message is shown in Table 5.

TABLE 5 Field Size Notes Global Header 152-bit  For (j=0; j<Num Records;j++) {  MSS unique identifier 48-bit 48-bit unique identifier used byMSS (as provided by the MSS or by the I-am-host-of message)  EstimatedTime to HO 16-bit In milliseconds, relative to the time stamp. A valueof 0 indicates that the estimated time is unknown.  Required BW  8-bitBandwidth which is required by MSS (to guarantee minimum packet datatransmission)  For (i=0; i<Num_SFID_Records; i++) {   SFID 32-bit   For(i=0; i<Num_QoS_Records;i++) {  Required QoS Variable 11.13 QoSParameter definition encodings that in combination define anAdmittedQoSParamSet specific to the SFIC  }  } } Security field TBD Ameans to authenticate this message

As shown in Table 5, the HO_PRE_NOTIFICATION message includes aplurality of IEs, i.e., a Global Header which is commonly included inmessages exchanged between BSs in a backbone network, an MSS ID of theMSS 200 that desires to be handed over to the first target BS 220 or thesecond target BS 230, an Estimated Time to HO indicating an estimatedtime at which the MSS 200 will start handover, a Required BW indicatinginformation on a bandwidth for which the MSS 200 requests a target BSwhich will become a new serving BS, an SFID indicating an ID of aservice flow that the MSS 200 is receiving, and a Required QoSindicating information on a service level for each SFID. The bandwidth(BW) and the service level (QoS) requested by the MSS 200 are equal tothe predicted service level information recorded in the MOB_MSSHO_REQmessage described in Table 4.

A format of the general Global Header commonly included in messagesexchanged between BSs in a backbone network, like theHO_PRE_NOTIFICATION message, is shown in Table 6.

TABLE 6 Field Size Notes Message Type = ?  8-bit Sender BS-ID 48-bitBase station unique identifier (Same number as that broadcasted on theDL-MAP message) Target BS-ID 48-bit Base station unique identifier (Samenumber as that broadcasted on the DL-MAP message) Time Stamp 32-bitNumber of milliseconds since midnight GMT (set to 0xffffffff to ignore)Num Records 16-bit Number of MSS identity records

As shown in Table 6, the Global Header includes a plurality of IEs,i.e., a Message Type indicating a type of a transmission message, aSender BS-ID indicating a transmission BS that transmits thetransmission message, a Target BS-ID indicating a reception BS thatreceives the transmission message, and a Num Records indicating thenumber of MSSs corresponding to records included in the transmissionmessage.

Upon receiving the HO_PRE_NOTIFICATION messages from the serving BS 210,the first target BS 220 and the second target BS 230 transmitHO_PRE_NOTIFICATION_RESPONSE messages to the serving BS 210 in responseto the HO_PRE_NOTIFICATION messages in Steps 229 and 231. A format ofthe HO_PRE_NOTIFICATION_RESPONSE message is shown in Table 7.

TABLE 7 Field Size Notes Global Header 152-bit  For (j=0; j<Num Records;j++) {  MSS unique 48-bit  48-bit unique identifier  identifier used byMSS (as provided by the MSS or by the I-am-host-of message)  BWEstimated 8-bit Bandwidth which is provided by BS (to guarantee minimumpacket data transmission) TBD how to set this field  QoS Estimated 8-bitQuality of Service level Unsolicited Grant Service (UGS) Real-timePolling Service (rtPS) Non-real-time Polling Service (nrtPS) Best Effort} Security Field TBD A means to authenticate this message

As shown in Table 7, the HO_PRE_NOTIFICATION_RESPONSE message includes aplurality of IEs, i.e., a Global Header which is commonly included inmessages exchanged between BSs in a backbone network, an MSS unique IDof the MSS 200 that desires to be handed over to the target BSs, andbandwidth and service level information for indicating a bandwidth and aservice level supportable by the target BSs to which the MSS 200 ishanded over.

Upon receiving the HO_PRE_NOTIFICATION_RESPONSE messages from the firsttarget BS 220 and the second target BS 230, the serving BS 210 analyzesthe HO_PRE_NOTIFICATION_RESPONSE messages received from the first targetBS 220 and the second target BS 230, and selects a target BS that canoptimally support the bandwidth and service level requested by the MSS200 after handover, as a final target BS to which the MSS 200 will behanded over. For example, if it is assumed that a service levelsupportable by the first target BS 220 is lower than the service levelrequested by the MSS 200 and a service level supportable by the secondtarget BS 230 is higher than or equal to the service level requested bythe MSS 200, the serving BS 210 selects the second target BS 230 as afinal target BS to which the MSS 200 will be handed over. Therefore, theserving BS 210 transmits a HO_CONFIRM message to the second target BS230 in response to the HO_PRE_NOTIFICATION_RESPONSE message in Step 233.A format of the HO_CONFIRM message is shown in Table 8.

TABLE 8 Field Size Notes Global Header 152-bit  For (j=0; j<Num Records;j++) {  MSS unique 48-bit  48-bit universal MAC address of the identifier MSS (as provided to the BS on the RNG-REQ message)  BWEstimated 8-bit Bandwidth which is provided by BS (to guarantee minimumpacket data transmission) TBD how to set this field  QoS Estimated 8-bitQuality of Service Level Unsolicited Grant Service (UGS) Real-timePolling Service (rtPS) Non-real-time Polling Service (nrtPS) Best EffortService (BE) } Security field TBD A means to authenticate this message

As shown in Table 8, the HO_CONFIRM message includes a plurality of IEs,i.e., a Global Header which is commonly included in messages exchangedbetween BSs in a backbone network as described with reference to Table6, an MSS ID of the MSS 200 that desires to be handed over to theselected target BS, and bandwidth and service level information forindicating a bandwidth and a service level supportable by the selectedtarget BS to which the MSS 200 is handed over.

In addition, the serving BS 210 transmits a Mobile BS Handover Response(MOB_BSHO_RSP) message to the MSS 200 in response to the MOB_MSSHO_REQmessage in Step 235. Herein, the MOB_BSHO_RSP message includesinformation on a target BS to which the MSS 200 will be handed over. Aformat of the MOB_BSHO_RSP message is shown in Table 9.

TABLE 9 Syntax Size Notes MOB_BSHO_RSP_Message_Format( ) {  ManagementMessage Type = 54 8 bits  Estimated HO time 8 bits  For (j=0;j<N_Recommended; j++) { Neighbor base stations shall be presented in anorder such that the first presented is the one most recommended and thelast presented is the least recommended. N_Recom- mended can be derivedfrom the known length of the message.   Neighbor BS-ID 48 bits   servicelevel prediction 8 bits  } }

As shown in Table 9, the MOB_BSHO_RSP message includes a plurality ofIEs, i.e., a Management Message Type indicating a type of a transmissionmessage, an Estimated HO time indicating an estimated time at which ahandover process will start, and information on target BSs selected bythe serving BS. In addition, an N_Recommended in the MOB_BSHO_RSPmessage indicates the number of target BSs satisfying the bandwidth andservice level requested by the MSS 200, among the target BSs in thecandidate target BS list. The MOB_BSHO_RSP message includes IDs fortarget BSs indicated by the N_Recommended, and a predicted service levelsupportable to the MSS 200 by the target BSs.

Although only the information on one target BS of the second target BS230 among the target BSs existing in the candidate target BS list isfinally included in the MOB_BSHO_RSP message in FIG. 2, if there areseveral target BSs satisfying the bandwidth and service level requestedby the MSS 200 among the target BSs existing in the candidate target BSlist, information on the several target BSs is included in theMOB_BSHO_RSP message.

Upon receiving the MOB_BSHO_RSP message, the MSS 200 analyzesN_Recommended information included in the received MOB_BSHO_RSP message,and selects a target BS to which it will be handed over based on theanalysis result.

After selecting the target BS, the MSS 200 transmits a Mobile HandoverIndication (MOB_HO_IND) message to the serving BS 210 in response to theMOB_BSHO_RSP message in Step 237. A format of the MOB_HO_IND message isshown in Table 10.

TABLE 10 Syntax Size Notes MOB_HO_IND_Message_Format( ) {  ManagementMessage Type = 56  8 bits  reserved  6 bits Reserved; shall be set tozero  HO_IND_type  2 bits 00: Serving BS released 01: HO cancel 10: HOreject 11: reserved  Target_BS_ID 48 bits Applicable only whenHO_IND-type is set to 00.  HMAC Tuple 21 bytes See 11.4.11 }

As shown in Table 10, the MOB_HO_IND message includes a plurality ofIEs, i.e., a Management Message Type indicating a type of a transmissionmessage, a HO_IND type indicating whether the MSS 200 has determined,canceled, or rejected handover to the selected final target BS, aTarget_BS_ID indicating an ID of the selected final target BS when theMSS 200 determines the handover, and a HMAC (Hashed MessageAuthentication Code) Tuple used for authentication of the MOB_HO_INDmessage. The MSS 200 transmits a MOB_HO_RSP message with HO_IND_type=00when it has determined to perform handover to the final target BS,transmits a MOB_HO_RSP message with HO_IND_type=01 when it hasdetermined to cancel the handover to the final target BS, and transmitsa MOB_HO_RSP message with HO_IND_type=10 when it has determined toreject the handover to the final target BS. Upon receiving theMOB_HO_IND message with HO_IND_type=10, the serving BS 210 updates thecandidate target BS list and retransmits a MOB_BSHO_RSP message with thecandidate target BS list to the MSS 200.

Upon receiving the MOB_HO_IND message with HO_IND_type=00, the servingBS 210 recognizes that the MSS 200 will perform handover to the targetBS included in the MOB_HO_IND message, i.e., the second target BS 230,and releases a connection currently set up to the MSS 200 or retains theconnection set up to the MSS 200 for a predetermined time until itreceives a report indicating completion of the handover process from thetarget BS finally selected by the MSS 200, i.e., the second target BS230, in Step 239.

After transmitting the MOB_HO_IND message to the serving BS 210, the MSS200 performs the remaining handover operation with the second target BS230.

FIG. 3 is a signaling diagram illustrating a handover process initiatedat the request of a BS in a conventional IEEE 802.16e communicationsystem. However, before a description of FIG. 3 is given, it should benoted that the handover initiated at the request of a BS occurs when theBS requires load sharing for dispersing its own load to neighbor BSs dueto its excessive load, or when it is necessary to cope with a variationin an uplink state of an MSS.

Referring to FIG. 3, a serving BS 310 transmits a MOB_NBR_ADV message toan MSS 300 in Step 311. The MSS 300 can acquire information on neighborBSs by receiving the MOB_NBR_ADV message.

If the serving BS 310 detects a need for handover of the MSS 300 that itis currently managing in Step 313, it transmits HO_PRE_NOTIFICATIONmessages to neighbor BSs in Steps 315 and 317. Herein, theHO_PRE_NOTIFICATION message includes information on a bandwidth andservice level that should be supported by a target BS, which will becomea new serving BS of the MSS 300. Additionally, it is assumed in FIG. 3that the neighbor BSs of the serving BS 310 include two BSs, i.e., afirst target BS 320 and a second target BS 330.

Upon receiving the HO_PRE_NOTIFICATION messages, the first target BS 320and the second target BS 330 transmit HO_PRE_NOTIFICATION_RESPONSEmessages to the serving BS 310 in response to the HO_PRE_NOTIFICATIONmessages, respectively, in Steps 319 and 321. TheHO_PRE_NOTIFICATION_RESPONSE message includes ACK/NACK indicating if thetarget BSs can perform handover requested by the serving BS 310, andinformation on a bandwidth and service level supportable to the MSS 300.

Upon receiving the HO_PRE_NOTIFICATION_RESPONSE messages from the firsttarget BS 320 and the second target BS 330, the serving BS 310 selectstarget BSs that can support the bandwidth and service level requested bythe MSS 300. For example, if it is assumed that a service levelsupportable by the first target BS 320 is lower than the service levelrequested by the MSS 300 and a service level supportable by the secondtarget BS 330 is higher than or equal to the service level requested bythe MSS 300, the serving BS 310 selects the second target BS 330 as atarget BS to which the MSS 300 can be handed over.

After selecting the second target BS 330 as a candidate target BS, theserving BS 310 transmits a Mobile BS Handover Request (MOB_BSHO_REQ)message including the updated candidate target BS list to the MSS 300 inStep 323. Herein, the candidate target BS list can include a pluralityof target BSs. A format of the MOB_BSHO_REQ message is shown in Table11.

TABLE 11 Syntax Size Notes MOB_BHSO_REQ_Message_Format( ) {  ManagementMessage Type = 52 8 bits  For (j=0; j<N_Recommended; j++) { N_Recom-mended can be derived from the known length of the message   NeighborBS-ID 48 bits    Service level prediction 8 bits  } }

As shown in Table 11, the MOB_BSHO_REQ message includes a plurality ofIEs, i.e., a Management Message Type indicating a type of a transmissionmessage and information on the target BSs selected by the serving BS310. In Table 11, an N_Recommended indicates the number of neighbor BSsselected as candidate target BSs by the serving BS 310, and theMOB_BSHO_REQ message includes a Neighbor BS-ID indicating IDs for theneighbor BSs indicated by the N_Recommended, and information on abandwidth and service level supportable to the MSS 300 by the neighborBSs.

Upon receiving the MOB_BSHO_REQ message, the MSS 300 recognizes thathandover has been requested by the serving BS 310, and selects a finaltarget BS to which it will perform handover, based on the N_Recommendedinformation included in the MOB_BSHO_REQ message. Before selecting thefinal target BS, if the MSS 300 desires to scan CINRs of the pilotchannel signals transmitted from the serving BS 310 and the neighborBSs, the MSS 300 transmits a MOB_SCN_REQ message to the serving BS 310in Step 325. Because a time at which the MSS 300 makes a scan request isnot directly related to a CINR scanning operation for the pilot channelsignals, a detailed description thereof will not be given herein.

Upon receiving the MOB_SCN_REQ message, the serving BS 310 transmits aMOB_SCN_RSP message including scanning information based on which theMSS 300 will perform scanning, to the MSS 300 in Step 327. Uponreceiving the MOB_SCN_RSP message including the scanning information,the MSS 300 performs CINR scanning on the pilot channel signals receivedfrom neighbor BSs acquired through reception of the MOB_NBR_ADV message,candidate target BSs acquired through reception of the MOB_BSHO_REQmessage, and the serving BS 310, according to parameters, i.e., ScanDuration, included in the MOB_SCN_RSP message, in Step 329.

After selecting its final candidate target BS, the MSS 300 transmits aMobile MSS Handover Response (MOB_MSSHO_RSP) to the serving BS 310 inresponse to the MOB_BSHO_REQ message in Step 331. A format of theMOB_MSSHO_RSP message is shown in Table 12.

TABLE 12 Syntax Size Notes MOB_MSSHO_RSP_Message_Format( ) {  ManagementMessage Type = 54 8 bits  Estimated HO time 8 bits  For (j=0;j<N_Recommended; j++) { N_Recom- mended can be derived from the knownlength of the message   Neighbor BS-ID 48 bits    BS S/(N+1) 8 bits  } }

As shown in Table 12, the MOB_MSSHO_RSP includes a plurality of IEs,i.e., a Management Message Type indicating a type of a transmissionmessage, an Estimated HO time indicating an estimated time at which thehandover process will start, and information on the target BSs selectedby the MSS 310. In Table 12, an N_Recommended indicates the number ofneighbor BSs selected as candidate target BSs by the MSS 300, and theMOB_MSSHO_RSP message includes a Neighbor BS-ID indicating IDs for theneighbor BSs indicated by the N_Recommended, and information on aservice level supportable to the MSS 300 by the neighbor BSs.

The serving BS 310 transmits a HO_CONFIRM message to the neighbor BSselected as the final target BS by the MSS 300 in response to theHO_PRE_NOTIFICATION_RESPONSE message in Step 333. After selecting thefinal target BS, the MSS 300 transmits a MOB_HO_IND message withHO_IND_type=00 to the serving BS 310 in Step 335.

Upon receiving the MOB_HO_IND message with HO_IND_type=00, the servingBS 310 re-recognizes that the MSS 300 will perform handover to the finaltarget BS included in the MOB_HO_IND message, and then releases aconnection currently set up to the MSS 300 or retains the connection setup to the MSS 300 for a predetermined time, until it receives a reportindicating completion of the handover process from the finally selectedtarget BS, i.e., the second target BS 330, in Step 337.

After transmitting the MOB_HO_IND message to the serving BS 310, the MSS300 performs the remaining handover operation with the second target BS330.

FIG. 4 is a signaling diagram illustrating a network re-entry processperformed after a handover of an MSS in a conventional IEEE 802.16ecommunication system. Referring to FIG. 4, as an MSS 400 changes itsconnection to a final target BS 450, acquires downlink synchronizationwith the final target BS 450, and receives a downlink_MAP (DL_MAP)message from the final target BS 450 in Step 411. Herein, the DL_MAPmessage includes parameters related to a downlink of the final target BS450.

Further, the MSS 400 receives an uplink_MAP (UL_MAP) message from thefinal target BS 450 in Step 413. The UL_MAP message includes parametersrelated to an uplink of the final target BS 450, and includes a Fast ULRanging IE allocated to support fast UL ranging of the MSS 400 whosehandover is being performed by the final target BS 450. The final targetBS 450 allocates the Fast UL Ranging IE to the MSS 400 to minimize apossible delay caused by handover. Therefore, the MSS 400 can performinitial ranging with the final target BS 450 on a contention-free basisaccording to the Fast UL Ranging IE. A format of the Fast UL Ranging IEincluded in the UL_MAP message is shown in Table 13.

TABLE 13 Syntax Size Notes Fast_UL_ranging IE {  Extended UIUC 4 bits MAC address 48 bits  MSS MAC address as provided on the RNG_REQ messageon initial system entry  UIUC 4 bits UIUC ≠ 15. A four-bit code used todefine the type of uplink access and the burst type associated with thataccess.  OFDM Symbol offset 10 bits  The offset of the OFDM symbol inwhich the burst starts, the offset value is defined in units of OFDMsymbols and is relevant to the Allocation Start Time field given in theUL-MAP message.  Subchannel offset 6 bits The lowest index OFDMAsubchannel used for carrying the burst, starting from subchannel 0.  No.OFDM symbols 10 bits  The number of OFDM symbol that are used to carrythe UL Burst  No. Subchannels 6 bits The number of OFDMA subchannelswith subsequent indexes, used to carry the burst.  Reserved 4 bits }

In Table 13, Fast_UL_ranging_IE( ) includes a Medium Access Control(MAC) address of an MSS that will be provided with ranging opportunity,an Uplink Interval Usage Code (UIUC) providing information on a field inwhich a start offset value for the Fast_UL_ranging_IE( ) is recorded, anoffset of and the number of symbols in a contention-free-based rangingopportunity interval allocated to the MSS 400, and the number ofsubchannels. A MAC address of the MSS 400 has been reported to the finaltarget BS 450 through messages exchanged between a serving BS and atarget BS in a backbone network in the handover process described withreference to FIGS. 2 and 3, e.g., theHO_PRE_NOTIFICATION/HO_PRE_NOTIFICATION_RESPONSE/HO_CONF IRM messages.

Upon receiving the UL_MAP message, the MSS 400 transmits a RangingRequest (RNG_REQ) message to the final target BS 450 according to theFast UL Ranging IE in Step 415. Upon receiving the RNG_REQ message, thefinal target BS 450 transmits a Ranging Response (RNG_RSP) messageincluding information used for correcting frequency, time andtransmission power for the ranging, to the MSS 400 in Step 417.

After completing the initial ranging, the MSS 400 and the final targetBS 450 perform a re-authorization operation on the MSS 400 (MSSRE-AUTHORIZATION) in Step 419. In the re-authorization operation, ifthere is no change in security context exchanged between an old (orformer) serving BS of the MSS 400 and the final target BS 450, the finaltarget BS 450 uses the security context intact. A format of an MSSInformation Response (MSS_INFO_RSP) message, which is a backbone networkmessage for providing security context information of the MSS 400, isshown in Table 14.

TABLE 14 Fields Size Notes Global Header 152-bit  For (j=0; j<NumRecords; j++) {  MSS unique identifier 48-bit 48-bit unique identifierused by MSS (as provided by the MSS or by the I-am-host-of message) N_NSIE Number of Network Service Information Elements  For (k=0;k<N_NSIE; k++) {   Field Size 16-bit Size, in bytes, of TLV encodedinformation field below   TLV encoded information Variable TLVinformation as allowed on a DSA-REQ MAC message  }  N_SAIE Number ofSecurity Associated Information Elements  For (k=0; k<N_SAIE; k++) {  Field Size 16-bit Size, in bytes, of TLV encoded information fieldbelow   TLV encoded information Variable TLV information as allowed on aPKM-xxx MAC message  }  N_MSS_CAP Number of MSS Capabilities  For (k=0;k<N_MSS_CAP; k++) {   Field Size 16-bit Size, in bytes, of TLV encodedinformation field below   TLV encoded information Variable TLVinformation as allowed on a SBC-REQ MAC message  } } Security field TBDA means to authenticate this message

In Table 14, the MSS_INFO_RSP message includes ID information of an MSSregistered in a serving BS, security context information such asSecurity Association Information for each MSS, network serviceinformation for each MSS, and capability information of each MSS.

When the re-authentication operation for the final target BS 450 and theMSS 400 is completed, the MSS 400 transmits a Registration Request(REG_REQ) message to the final target BS 450 in Step 421. The REG_REQmessage includes registration information of the MSS 400. The finaltarget BS 450 transmits a Registration Response (REG_RSP) message to theMSS 400 in response to the REG_REQ message in Step 423. Herein, thefinal target BS 450 can recognize the MSS 400 as an MSS that has beenhanded over thereto, by detecting registration information of the MSS400 included in the REG_REQ message received from the MSS 400.Accordingly, the final target BS 450 maps connection information in theold serving BS of the MSS 400 to connection information in the finaltarget BS 450, and transmits to the MSS 400 the REG_RSP messageincluding TLV values based on which a service flow that can be actuallyprovided in the final target BS can be reset. A format of the TLVincluding mapping information for connection setup in the serving BS andthe final target BS 450 is shown in Table 15.

TABLE 15 Type Length Name (1 byte) (1 byte) Value (Variable length)New_CID 2.1 2 New CID after handover to new BS Old_CID 2.2 2 Old CIDbefore handover to old BS Connection 2.3 Variable If any of the serviceflow Info parameters change, then those service flow parameters and CSparameter encoding TLVs that have changed will be added. Connection_Infois a compound TLV value that encapsulates the Service Flow Parametersand the CS Parameter that have changed for the service. All the rulesand settings that apply to the parameters when used in the DSC- RSPmessage apply to the contents encapsulated in this TLV.

In Table 15, TLV included in the REG_RSP message transmitted to the MSS400 provides CID information used in the old serving BS, before handoverof the MSS 400, and CID information to be used in the final target BS450, after handover of the MSS 400. In addition, when the final targetBS 450 provides a service that is different from the service flowprovided by the old serving BS before handover, the TLV includesinformation on the changed service parameters.

After completing the network re-entry process with the final target BS450, the MSS 400 performs a normal communication service through thefinal target BS 450 in Step 425.

As described above, in the IEEE 802.16e communication system, in ahandover process initiated at the request of an MSS, the MSS determinesa need for handover by measuring a change in downlink channel through ascanning process and transmits a handover request message to a servingBS. In response, the serving BS receives service level predictioninformation for neighbor BSs recommended by the MSS and delivers theservice level prediction information back to the MSS. Then the MSS canselect a final target BS based on CINRs of downlink channels from theneighbor BSs and service level information of the neighbor BSs. The CINRmeasurement is performed before the MSS determines handover, and amessage exchange for predicting a service level supportable in abackbone network is performed after the MSS determines to handover.

However, in a handover process initiated at the request of a BS, aserving BS, after determining a need for handover, should exchangemessages for service level prediction with all neighbor BSs. An MSSreceiving a MOB_BSHO_REQ message including a recommended-BS list shouldperform a scanning operation to select a final target BS among therecommended BSs. Therefore, compared with the handover process initiatedat the request of an MSS, the handover process initiated at the requestof a BS performs both the service level measurement and the scanningoperation after the handover is determined.

It can be expected that the number of BSs recommended by an MSS throughCINR values in the handover process initiated at the request of an MSSis less than the number of BSs recommended through service levelprediction in the handover process initiated at the request of a BSbecause a time for which the MSS performs CINR scanning on each ofneighbor BSs is short. Accordingly, compared with the handover processinitiated at the request of an MSS, the handover process initiated atthe request of a BS requires a longer processing time.

Further, a BS sends a handover request to an MSS for several reasons,especially when the BS determines that handover of the MSS is urgent.Therefore, there is a demand for a method capable of performing ahandover over a shorter time by improving the handover process initiatedat the request of a BS, which requires a longer processing time comparedwith the handover process initiated at the request of an MSS.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a systemand method, in which a mobile subscriber station (MSS) performs fasthandover at the request of a base station (BS) in a broadband wirelessaccess (BWA) communication system.

According to an aspect of the present invention, there is provided aBroadband Wireless Access (BWA) communication system. The systemincludes: a Mobile Subscriber Station (MSS); and a serving Base Station(BS) for transmitting, to the MSS, a handover request message includinginformation on at least one recommended BS to which the MSS can performhandover, among a plurality of neighbor BSs of the serving BS, and aninformation field indicating whether a network assisted handover issupported, receiving, from the MSS, a handover indication messageincluding an IDentifier (ID) field of a target BS, receiving informationindicating a completion of a handover process of the MSS from a selectedtarget BS determined by the MSS among the at least one recommended BS,and transmitting a notification for ordering a release of a resourceallocated to the MSS, to the other at least one recommended BS,excluding the selected target BS from the at least one recommended BS.The network assisted handover indicates that the MSS may perform ahandover to the selected target BS among the at least one recommended BSwithout notifying the serving BS of the selected target BS. The ID fieldof the handover indication message is set to a predefined value thatdoes not indicate a certain target BS, when the information fieldindicates the network assisted handover is supported.

According to another aspect of the present invention, there is provideda Broadband Wireless Access (BWA) communication system. The systemincludes: a serving Base Station (BS); and a Mobile Subscriber Station(MSS) for receiving, from the serving BS, a handover request messageincluding information on at least one recommended BS to which the MSScan perform handover, among a plurality of neighbor BSs of the servingBS, and an information field indicating whether a network assistedhandover is supported, transmitting, to the serving BS, a handoverindication message including an IDentifier (ID) field of a target BS,receiving, from the at least one recommended BS, a ranging resource forthe network assisted handover of the MSS, transmitting, to a selectedtarget BS among the at least one recommended BS, a ranging requestmessage, and receiving, from the selected target BS, a ranging responsemessage. The network assisted handover indicates that the MSS mayperform a handover to the selected target BS among the at least onerecommended BS without notifying the serving BS of the selected targetBS. The ID field of the handover indication message is set to apredefined value that does not indicate a certain target BS, when theinformation field indicates the network assisted handover is supported.

According to further another aspect of the present invention, there isprovided an apparatus for performing a handover of a Mobile SubscriberStation (MSS) in a serving Base Station (BS) of a Broadband WirelessAccess (BWA) communication system. The apparatus includes: a transmitterfor transmitting, to the MSS, a handover request message includinginformation on at least one recommended BS to which the MSS can performhandover, from among a plurality of neighbor BSs of the serving BS, andan information field indicating whether a network assisted handover issupported; and a receiver for receiving, from the MSS, a handoverindication message including an IDentifier (ID) field of a target BS,and receiving information indicating a completion of a handover processof the MSS from a selected target BS determined by the MSS among the atleast one recommended BS. The transmitter transmits a notification forordering a release of a resource allocated to the MSS, to the other atleast one recommended BS, excluding the selected target BS from the atleast one recommended BS. The network assisted handover indicates thatthe MSS may perform a handover to the selected target BS among the atleast one recommended BS without notifying the serving BS of theselected target BS. The ID field of the handover indication message isset to a predefined value that does not indicate a certain target BS,when the information field indicates the network assisted handover issupported.

According to yet another aspect of the present invention, there isprovided an apparatus for performing a handover in a Mobile SubscriberStation (MSS) of a Broadband Wireless Access (BWA) communication system.The apparatus includes: a receiver for receiving, from a serving BaseStation (BS), a handover request message including information on atleast one recommended BS to which the MSS can perform handover, fromamong a plurality of neighbor BSs of the serving BS, and an informationfield indicating whether a network assisted handover is supported; and atransmitter for transmitting, to the serving BS, a handover indicationmessage including an IDentifier (ID) field of a target BS. The receiverreceives, from the at least one recommended BS, ranging resource for thenetwork assisted handover of the MSS, the transmitter transmits, to aselected target BS among the at least one recommended BS, a rangingrequest message, and the receiver receives, from the selected target BS,a ranging response message. The network assisted handover indicates thatthe MSS is permitted to perform a handover to the selected target BSamong the at least one recommended BS without notifying the serving BSof the selected target BS. The ID field of the handover indicationmessage is set to a predefined value that does not indicate a certaintarget BS, when the information field indicates the network assistedhandover is supported.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagram schematically illustrating a conventional IEEE802.16e communication system;

FIG. 2 is a signaling diagram illustrating a handover process initiatedby an MSS in a conventional IEEE 802.16e communication system;

FIG. 3 is a signaling diagram illustrating a handover process initiatedby a BS in a conventional IEEE 802.16e communication system;

FIG. 4 is a signaling diagram illustrating a network re-entry processperformed after a handover of an MSS in a conventional IEEE 802.16ecommunication system;

FIG. 5 is a signaling diagram illustrating a fast handover processduring a handover initiated by a BS in an IEEE 802.16e communicationsystem according to an embodiment of the present invention;

FIG. 6 is a flowchart illustrating a fast handover process of an MSSinitiated by a BS in an IEEE 802.16e communication system according toan embodiment of the present invention;

FIG. 7 is a flowchart illustrating an operation of a serving BS during ahandover initiated by a BS in an IEEE 802.16e communication systemaccording to an embodiment of the present invention; and

FIG. 8 is a flowchart illustrating an operation of a target BS during ahandover process initiated by a BS in an IEEE 802.16e communicationsystem according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will now be described indetail herein below with reference to the annexed drawings. In thefollowing description, a detailed description of known functions andconfigurations incorporated herein has been omitted for conciseness.

The present invention proposes a scheme for a fast handover of a mobilesubscriber station (MSS) initiated at a handover request of a basestation (BS) in an Institute of Electrical and Electronics Engineers(IEEE) 802.16e communication system, which is a broadband wirelessaccess (BWA) communication system. Herein, the IEEE 802.16ecommunication system is a BWA communication system using an OrthogonalFrequency Division Multiplexing (OFDM) scheme and/or an OrthogonalFrequency Division Multiple Access (OFDMA) scheme. The IEEE 802.16ecommunication system, as it uses the OFDM/OFDMA scheme, can supporthigh-speed data transmission by transmitting physical channel signalsusing a plurality of subcarriers. Further, the IEEE 802.16ecommunication system supports a multicell structure to support mobilityof MSSs.

FIG. 5 is a signaling diagram illustrating a signaling procedure betweenan MSS and BSs for supporting fast handover of the MSS initiated by a BSin an IEEE 802.16e communication system according to an embodiment ofthe present invention. Referring to FIG. 5, if a serving BS 540 detectsa need for a handover of an MSS 500 that it is managing in Step 511, theserving BS 540 starts a handover process initiated at the request of aBS. Accordingly, the serving BS 540 transmits HO_PRE_NOTIFICATIONmessages to neighbor BSs to which the MSS 500 will possibly be handedover in Steps 513, 515, and 517. It is assumed in FIG. 5 that theneighbor BSs of the serving BS 540 includes three BSs, i.e., a firsttarget BS 560, a second target BS 580, and a third target BS 590.

Upon receiving the HO_PRE_NOTIFICATION message, each of the target BSstransmit a HO_PRE_NOTIFICATION_RESPONSE message to the serving BS 540 inSteps 519, 521, and 523. Thereafter, the serving BS 540 selects a targetBS or target BSs that can support a bandwidth and a service levelrequested by the MSS 500. For example, if it is assumed that a servicelevel supportable by the first target BS 560 and the second target BS580 is higher than or equal to the service level requested by the MSS500 and a service level supportable by the third target BS 590 is lowerthan the service level requested by the MSS 500, the serving BS 540selects the first target BS 560 and the second target BS 580 ascandidate target BSs.

The serving BS 540 creates a recommended list using the candidate targetBSs, and transmits a MOB_BSHO_REQ message including the recommended listto the MSS 500 in Step 527. A format of the MOB_BSHO_REQ message used inthis embodiment is shown in Table 16.

TABLE 16 Syntax Size Notes MOB_BSHO_REQ_Message_Format( ) {  ManagementMessage Type = 52  reserved 7 bits  HO_option 1 bit  0: Normal 1: Fast(Network Assisted)  For (j=0; j<N_Recommended; j++) {   Neighbor BS-ID48 bits    Service level prediction 8 bits  } }

As shown in Table 16, compared with the conventional MOB_BSHO_REQmessage of Table 11, the redefined MOB_BSHO_REQ message further includesa 1-bit HO_option field and a 7-bit reserved field. If a HO_option valueof the redefined MOB_BSHO_REQ message is set to ‘0’ (HO_option=0), theMSS 500 operates as when the conventional MOB_BSHO_REQ message isreceived, i.e., the procedure illustrated in FIG. 3. However, if theHO_option value is set to ‘1’ (HO_option=1), the MSS 500 performs a fast(network assisted) handover process as proposed in the presentinvention.

The serving BS 540 transmits HO_CONFIRM messages of Table 8 to thetarget BSs included in the recommended list, i.e., the first and secondtarget BSs 560 and 580, in Steps 525 and 529. Upon receiving theHO_CONFIRM messages, the first and second target BSs 560 and 580 eachreserve a Fast UL Ranging IE for the MSS 500, and transmits a UL_MAPmessage including the Fast UL Ranging IE to the MSS 500 in Steps 535 and537.

Optionally, in addition to the HO_CONFIRM messages, the serving BS 540can transmit information needed for an operation of the MSS 500, storedtherein, to the first and second target BSs 560 and 580 using backbonemessages. Herein, the information needed for an operation of the MSS 500refers to handover process optimization information such as serviceinformation of the MSS 500 and operational context information.Thereafter, the MSS 500, when it performs the handover to a BS includedin the recommended list, can perform ranging with the corresponding BSfor a fast ranging interval detected from a UL_MAP message. If a targetBS to which the MSS 500 will be handed over has already acquiredinformation on the MSS 500, the MSS 500 can perform a network re-entryprocess caused by handover more rapidly.

Although it is illustrated in FIG. 5 that the first and second targetBSs 560 and 580 each transmit the Fast UL Ranging IE, after the MSS 500performs carrier-to-interference and noise ratio (CINR) scanning onpilot channel signals from neighbor BSs in Step 533, the first andsecond target BSs 560 and 580 each can optionally transmit the Fast ULRanging IE immediately after receiving the HO_CONFIRM messages. That is,the first target BS 560 can transmit the Fast UL Ranging IE to the MSS500 after receiving the HO_CONFIRM message from the serving BS 540 inStep 525. Also, the second target BS 580 can transmit the Fast ULRanging IE to the MSS 500 after receiving the HO_CONFIRM message fromthe serving BS 540 in Step 529.

Upon receiving the MOB_BSHO_REQ message with HO_option=1, the MSS 500transmits a MOB_HO_IND message to the serving BS 540 in Step 531.Herein, a value recorded in a Target_BS_ID field of the MOB_HO_INDmessage can be either an identifier (ID) of a certain target BS or avalue predefined between the target BS and the MSS 500. For example,0x000000000000 or 0xfffffffffff can be used as the predefined value.When there are two or more recommended BSs for CINR scanning, the MSS500 can write the predefined value in the Target_BS_ID field of theMOB_HO_IND message.

The conventional MOB_HO_IND message was used when an MSS informs aserving BS of a selected final target BS to which it will be handedover. Conventionally, therefore, the serving BS must receive theMOB_HO_IND message to perform handover of the MSS. However, even when aserving BS fails to receive a MOB_HO_IND message defined in the systemsupporting fast handover of an MSS according to the present invention orthe MSS does not transmit the redefined MOB_HO_IND message to theserving BS, the present invention enables the MSS to perform a handoverbecause a value recorded in a Target_BS_ID field of the redefinedMOB_HO_IND message may not be an ID of a final target BS to which theMSS will perform handover.

After transmitting the MOB_HO_IND message, the MSS 500 performs CINRscanning on the BSs included in the recommended list in Step 533, andselects a best target BS (hereinafter referred to as a “new servingBS”). For example, if a CINR value for the second target BS 580 is thegreatest among the scanned BSs, the MSS 500 can select the second targetBS 580 as a new serving BS.

In a method proposed by the present invention, because the MSS 500 isnot required to transmit the MOB_MSSHO_RSP message including CINR valuesfor the recommended target BSs to the serving BS 540, after performingCINR scanning on pilot channel signals from the recommended target BSs,it is not necessary for the MSS 500 to perform CINR scanning on all BSsbelonging to the recommended list. For example, assuming that there is aCINR threshold based on which the MSS 500 can perform handover, the MSS500 first performs CINR scanning on the second target BS 580. If thescanning result value is sufficiently higher than the CINR threshold,such that a handover is possible, the MSS 500 is not required to performCINR scanning on the first target BS 560.

Therefore, the MSS 500 can read a Fast UL Ranging IE allocated theretoby checking a UL_MAP message broadcasted by the second target BS 580 andstart a network re-entry process by transmitting a Ranging Request(RNG_REQ) message to the second target BS 580.

In the network re-entry process, the MSS 500 returns to the serving BS540 after CINR scanning, acquires synchronization with the serving BS540, and performs a ranging process, thereby reducing a time requiredfor transmitting the MOB_MSSHO_RSP message and the MOB_HO_IND message.In addition, the MSS 500 starts the network re-entry process immediatelyafter early completion of CINR scanning, without performing CINRscanning on all target BSs included in the recommended list, therebycontributing to additional time saving.

After completing the CINR scanning, the MSS 500 receives the Fast ULRanging IE transmitted by the second target BS 580 that is selected as anew serving BS in Step 537, and performs a network re-entry process withthe second target BS 580 in Steps 539, 541, and 543.

Upon receiving a request for authentication on the MSS 500 from thesecond target BS 580, which is a new serving BS of the MSS 500, theserving BS 540 recognizes that the MSS 500 is performing handover to thesecond target BS 580. The serving BS 540 transmits a handover withdraw(HO_WITHDRAW) message to each of the remaining recommended BSs in therecommended list (, except the second target BS 580, which is the newserving BS, in Step 545. A format of the HO_WITHDRAW message is shown inTable 17.

TABLE 17 Field Size Notes Global Header 152-bit For (j=0; j<Num_Records;j++) {  MSS unique identifier  48-bit 48-bit general purpose MAC addressfor MSS } Security field TBD Used for authenticating this message

Because it is assumed in FIG. 5 that the recommended list includes thefirst target BS 560 and the second target BS 580, the HO_WITHDRAWmessage is transmitted only to the first target BS 560.

Upon receiving the HO_WITHDRAW message, the first target BS 560 releasesa reserved uplink resource by deleting the Fast UL Ranging IE for theMSS 500. The serving BS 540 can transmit the HO_WITHDRAW message at anytime after the time at which it recognizes that the MSS 500 has selectedthe second target BS 580 as its final target BS.

For example, the serving BS 540 can transmit the HO_WITHDRAW messageusing an authentication-related HMAC Tuple field included in theMOB_HO_IND message at the time when it responds to the request forauthentication on the MSS 500 from the second target BS 580.Alternatively, if the MSS 500 completes the network re-entry process tothe second target BS 580, which is its new serving BS, the serving BS540 can transmit the HO_WITHDRAW message, upon receiving a backbonemessage that the second target BS 580 transmits to inform neighbor BSsof the completed handover of the MSS 500.

Furthermore, if the first target BS 560 excluded from the handovertarget BSs fails to receive the RNG_REQ message from the MSS 500 for apredetermined time, the MSS 500 may release the Fast UL Ranging IEallocated thereto. Although the old serving BS 540 transmits theHO_WITHDRAW message using the method of releasing the Fast UL Ranging IEin the embodiment of the present invention, the message can betransmitted in another method.

FIG. 6 is a flowchart illustrating a handover process of an MSSinitiated by a BS in an IEEE 802.16e communication system according toan embodiment of the present invention. Referring to FIG. 6, in step611, the MSS receives a MOB_BSHO_REQ message for requesting handoverfrom a serving BS. In step 613, if a value recorded in a HO_option fieldincluded in the MOB_BSHO_REQ message is set to ‘1’, the MSS recognizesthat it can perform fast handover, and then performs steps 625 and thefollowing process, which is a fast handover process. However, if thevalue recorded in the HO_option field is set to ‘0’, the MSS performsstep 615 and the following process, which is the conventional handoverprocess.

In step 615, the MSS performs CINR scanning on each of pilot channelsignals from recommended BSs included in the MOB_BSHO_REQ message. Instep 617, the MSS transmits a MOB_MSSHO_RSP message to its serving BS.Herein, the MOB_MSSHO_RSP message includes the CINR values for thescanned BSs.

In step 619, the MSS selects a new serving BS among the scanned BSs. Forexample, the MSS can select a BS, a CINR value for which is greatest, asthe new serving BS. In step 621, the MSS transmits a MOB_HO_IND messageincluding a predefined value between the MSS and the serving BS. Herein,the MSS may not transmit the MOB_HO_IND message to the serving BS.

In step 633, the MSS acquires synchronization with the new serving BS,and acquires network re-entry process-related information by receiving aUL_MAP message. For example, the MSS can perform a ranging process withthe new serving BS on a contention-free basis by detecting a Fast ULRanging IE included in the UL_MAP message. In step 635, the MSS performsa network re-entry process with the selected target BS, i.e., the newserving BS, to resume a communication service.

Alternatively, in step 625, the MSS transmits a MOB_HO_IND message withHO_IND_type=0 and Target_BS_ID=‘predefined value’ to the serving BS.Herein, the predefined value means, for example, 0x000000000000 or0xffffffffffff other than one of IDs of BSs supporting fast handover.That is, the MSS can select a random BS among BSs supporting fasthandover and set the Target_BS_ID to an ID value of the selected BS.

In step 629, the MSS performs CINR scanning on the target BSsrecommended by the serving BS. In step 631, the MSS selects a newserving BS according to the scanning result, and then performs steps 633and 635. The MSS can select the new serving BS by performing CINRscanning on each of the recommended BSs.

Alternatively, the MSS can perform CINR scanning on a random recommendedBS and, if the scanning result satisfies a predetermined condition,select the random recommended BS as the new serving BS after stoppingCINR scanning on the remaining recommended BSs.

FIG. 7 is a flowchart illustrating an operation of a serving BS during ahandover initiated by a BS in an IEEE 802.16e communication systemaccording to an embodiment of the present invention. Referring to FIG.7, in step 711, the serving BS detects a need for handover of aparticular MSS. In step 713, the serving BS transmits aHO_PRE_NOTIFICATION message to each of neighbor BSs to determine if theneighbor BSs can support a service level requested by the MSS. In step715, the serving BS generates a recommended-BS list usingHO_PRE_NOTIFICATION_RESPONSE messages received from the neighbor BSs inresponse to the HO_PRE_NOTIFICATION message. In step 717, if the servingBS can support fast handover for the MSS, the serving BS proceeds tostep 727. However, if the serving BS cannot support fast handover forthe MSS, it proceeds to step 719.

In step 719, the serving BS transmits a MOB_BSHO_REQ message includingthe generate recommended-BS list and service level prediction (SLP)information of each of the recommended BSs to the MSS. In step 721, theserving BS receives an MOB_MSSHO_RSP message including CINR canningresult for each of the recommended BSs from the MSS. In step 723, theserving BS receives an MOB_HO_IND message including an ID of a target BSselected by the MSS, i.e., a new serving BS. In step 725, the serving BStransmits a HO_CONFIRM message to the new serving BS. In step 741, theserving BS releases information on a connection set up to the MSS orretains the connection information for a predetermined time.

In step 727, the serving BS transmits a MOB_BSHO_REQ message includingthe generated recommended-BS list and SLP information of each of therecommended BSs to the MSS. A HO_option field in the MOB_BSHO_REQmessage is set to ‘1’.

In step 729, the serving BS transmits a HO_CONFIRM message to each ofthe recommended BSs. In step 731, the serving BS receives a MOB_HO_INDmessage from the MSS. In step 733, the serving BS recognizes that theMSS has selected a new serving BS. As described above, a time at whichthe serving BS transmits a HO_WITHDRAW message is possible any timeafter the time at which it recognizes that the MSS has selected the newserving BS. Therefore, the serving BS can transmit the HO_WITHDRAWmessage using an authentication-related HMAC Tuple field included in theMOB_HO_IND message at the time when it responds to a request forauthentication on the MSS from the new serving BS selected by the MSS.

Alternatively, if the MSS completes a network re-entry process to thenew serving BS, the new serving BS transmits a backbone messageindicating the completed handover of the MSS to each of neighbor BSs.The serving BS can also transmit the HO_WITHDRAW message to BSs in therecommended-BS list, except the new serving BS, upon receiving thebackbone message. Therefore, in step 735, the serving BS selects BSs,not including the final target BS, among the BSs in the recommended-BSlist.

In step 737, the serving BS transmits the HO_WITHDRAW message to theselected BSs, i.e., target BSs. Upon receiving the HO_WITHDRAW message,the target BSs release a Fast UL Ranging IE allocated to the MSS, anddelete any handover process information of the MSS stored therein.

FIG. 8 is a flowchart illustrating an operation of a target BS duringhandover initiated by a BS in an IEEE 802.16e communication systemaccording to an embodiment of the present invention. Referring to FIG.8, in step 811, the target BS receives a HO_PRE_NOTIFICATION message forrequesting SLP for a particular MSS from a serving BS. In step 813, thetarget BS transmits a HO_PRE_NOTIFICATION_RESPONSE message to theserving BS in response to the HO_PRE_NOTIFICATION message. In step 815,the target BS receives a HO_CONFIRM message from the serving BS. In step817, the target BS transmits a UL_MAP message including a Fast ULRanging IE to the MSS.

In step 819, the target BS determines if it receives a HO_WITHDRAWmessage from the serving BS. If it is determined that the HO_WITHDRAWmessage is received, the target BS proceeds to step 821 where itreleases the Fast UL Ranging IE allocated to the MSS.

However, upon failure to receive the HO_WITHDRAW message, the target BSdetermines in step 823 if it receives an RNG_REQ message from the MSS.If it is determined that the RNG_REQ message is received, the target BSproceeds to step 825, where it transmits an RNG_RSP message to the MSSin response to the RNG_REQ message.

In step 827, the target BS performs a network re-entry process except aranging process, with the MSS.

Upon failure to receive the RNG_REQ message in step 823, the target BSproceeds to step 824, where it determines if a timer set for releasing aFast UL Ranging IE has expired. If it is determined that the timer hasexpired, the target BS proceeds to step 821, where it releases the FastUL Ranging IE. However, if the timer has not expired yet, the target BSreturns to step 819 where it awaits a HO_WITHDRAW message.

As can be understood from the foregoing description, a proposed BWAcommunication system using an OFDM/OFDMA scheme supports fast handoverto an MSS at the handover request of a serving BS to omit a part of theconventional handover process, thereby contributing to a reduction inhandover processing time. More specifically, it is possible to excludeone process in which the MSS transmits a handover response message tothe serving BS and another process in which the MSS acquiressynchronization with the serving BS and performs ranging with theserving BS after performing CINR scanning to transmit the handoverresponse message. The exclusion of these processes reduces the timerequired for the processes, thereby reducing the overall time requiredfor the handover process initiated by the BS.

While the present invention has been shown and described with referenceto certain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention as defined by the appended claims.

1. A Broadband Wireless Access (BWA) communication system comprising: aMobile Subscriber Station (MSS); and a serving Base Station (BS) fortransmitting, to the MSS, a handover request message includinginformation on at least one recommended BS to which the MSS can performhandover, among a plurality of neighbor BSs of the serving BS, and aninformation field indicating whether a network assisted handover issupported, receiving, from the MSS, a handover indication messageincluding an IDentifier (ID) field of a target BS, receiving informationindicating a completion of a handover process of the MSS from a selectedtarget BS determined by the MSS among the at least one recommended BS,and transmitting a notification for ordering a release of a resourceallocated to the MSS, to the other at least one recommended BS,excluding the selected target BS from the at least one recommended BS,wherein the network assisted handover indicates that the MSS may performa handover to the selected target BS among the at least one recommendedBS without notifying the serving BS of the selected target BS, andwherein the ID field of the handover indication message is set to apredefined value that does not indicate a certain target BS, when theinformation field indicates the network assisted handover is supported.2. The system of claim 1, wherein the serving BS receives anauthentication request for the MSS from the selected target BSdetermined by the MSS handover among the at least one recommended BS. 3.A Broadband Wireless Access (BWA) communication system comprising: aserving Base Station (BS); and a Mobile Subscriber Station (MSS) forreceiving, from the serving BS, a handover request message includinginformation on at least one recommended BS to which the MSS can performhandover, among a plurality of neighbor BSs of the serving BS, and aninformation field indicating whether a network assisted handover issupported, transmitting, to the serving BS, a handover indicationmessage including an IDentifier (ID) field of a target BS, receiving,from the at least one recommended BS, a ranging resource for the networkassisted handover of the MSS, transmitting, to a selected target BSamong the at least one recommended BS, a ranging request message, andreceiving, from the selected target BS, a ranging response message,wherein the network assisted handover indicates that the MSS may performa handover to the selected target BS among the at least one recommendedBS without notifying the serving BS of the selected target BS, andwherein the ID field of the handover indication message is set to apredefined value that does not indicate a certain target BS, when theinformation field indicates the network assisted handover is supported.4. The system of claim 3, wherein the selected target BS is determinedby measuring channel quality for each of the at least one recommended BSand determining a BS having the highest channel quality as the selectedtarget BS.
 5. The system of claim 3, wherein the MSS transmits, to eachof the at least one recommended BS, a handover message indicating thatthe MSS will perform the network assisted handover, when the informationfield indicates the network assisted handover is supported.
 6. Anapparatus for performing a handover of a Mobile Subscriber Station (MSS)in a serving Base Station (BS) of a Broadband Wireless Access (BWA)communication system, the apparatus comprising: a transmitter fortransmitting, to the MSS, a handover request message includinginformation on at least one recommended BS to which the MSS can performhandover, from among a plurality of neighbor BSs of the serving BS, andan information field indicating whether a network assisted handover issupported; and a receiver for receiving, from the MSS, a handoverindication message including an IDentifier (ID) field of a target BS,and receiving information indicating a completion of a handover processof the MSS from a selected target BS determined by the MSS among the atleast one recommended BS, wherein the transmitter transmits anotification for ordering a release of a resource allocated to the MSS,to the other at least one recommended BS, excluding the selected targetBS from the at least one recommended BS, wherein the network assistedhandover indicates that the MSS may perform a handover to the selectedtarget BS among the at least one recommended BS without notifying theserving BS of the selected target BS, and wherein the ID field of thehandover indication message is set to a predefined value that does notindicate a certain target BS, when the information field indicates thenetwork assisted handover is supported.
 7. The apparatus of claim 6,wherein the receiver receives an authentication request for the MSS fromthe selected target BS determined by the MSS handover among the at leastone recommended BS.
 8. An apparatus for performing a handover in aMobile Subscriber Station (MSS) of a Broadband Wireless Access (BWA)communication system, the apparatus comprising: a receiver forreceiving, from a serving Base Station (BS), a handover request messageincluding information on at least one recommended BS to which the MSScan perform handover, from among a plurality of neighbor BSs of theserving BS, and an information field indicating whether a networkassisted handover is supported; and a transmitter for transmitting, tothe serving BS, a handover indication message including an IDentifier(ID) field of a target BS, wherein the receiver receives, from the atleast one recommended BS, ranging resource for the network assistedhandover of the MSS, the transmitter transmits, to a selected target BSamong the at least one recommended BS, a ranging request message, andthe receiver receives, from the selected target BS, a ranging responsemessage, wherein the network assisted handover indicates that the MSS ispermitted to perform a handover to the selected target BS among the atleast one recommended BS without notifying the serving BS of theselected target BS, and wherein the ID field of the handover indicationmessage is set to a predefined value that does not indicate a certaintarget BS, when the information field indicates the network assistedhandover is supported.
 9. The apparatus of claim 8, wherein the selectedtarget BS is determined by measuring a channel quality for each of theat least one recommended BS and determining a BS having the highestchannel quality as the selected target BS.
 10. The apparatus of claim 8,wherein the transmitter transmits, to each of the at least onerecommended BS, a handover message indicating that the MSS will performthe network assisted handover, when the information field indicates thenetwork assisted handover is supported.